Manual of the Planes gives the Astral Plane the "No Gravity" trait - the 3.5 DMG does give it the directional one and technically might overrule that (but the Planar Handbook clearly disagrees).

I don't know what, if any, trait the 3.0 DMG gave to the Astral Plane.

IMHO you should probably assume that the PrC assumes that the Astral has no gravity?